1. Field of the Invention
This invention relates to an idling revolution number control valve which is provided at an intake passage of an internal combustion engine, and particularly to a linear solenoid which drives the valve by electromagnetic force.
2. Discussion of Background
Conventionally, almost all of the idling revolution number control valves for an internal combustion engine are driven by a solenoid utilizing electromagnetic force or by a motor. Copper wires are utilized for the electromagnetic coil which generates magnetomotive force in these electromagnetic actuators. Among these, especially, many of the idling revolution number control valve-with linear solenoid drivers, have an idling control function wherein the idling revolution number of an automobile is stabilized, an idling speed-up function wherein lowering of the idling revolution number is corrected corresponding to an electrical load or an air conditioner load, and a fast idling function wherein the idling revolution after the starting-up of the engine in cold temperature is maintained and warming-up of the engine is accelerated. Therefore, the required flow quantities of air are different under various conditions. It is important to flow the required quantity of air corresponding to these conditions, by performing an operation which complies with a direction from an electronic control unit (hereinafter ECU).
However, when a solenoid is utilized as a driver, its coil resistance is increased by the temperature elevation of the coil when electric current flows. When electric voltage corresponding to a battery voltage is applied to the solenoid to activate it, the coil temperature is elevated above normal, current flow is decreased, force generated by the solenoid is reduced, and a target flow quantity of air can not be achieved. For instance, a relationship between a solenoid terminal voltage and the coil resistance when the coil temperature is set at 23.degree. C. and 80.degree. C., is shown in the following equations. EQU I.sub.23 =E.sub.B /R.sub.23, EQU I.sub.80 =E.sub.B /R.sub.80,
where I.sub.23 is an average current at 23.degree. C., E.sub.B, the solenoid terminal voltage, R.sub.23, the coil resistance at 23.degree. C., I.sub.80, the average current at 80.degree. C., and R.sub.80, the coil resistance at 80.degree. C.
Furthermore, since R.sub.80 &gt;R.sub.23 from the above relationship, I.sub.80 &lt;I.sub.23. Since the magnetomotive force of the solenoid is determined by a turn number of the coil, N and the flow current, I, therefore, NI.sub.80 &lt;NI.sub.23. Since the force generated by the solenoid is reduced due to the above relationship, even under the same terminal voltage, the target flow quantity of air is not achieved.
Furthermore, in the linear solenoid valve of this kind, a duty control wherein the current to the electromagnetic coil intermittently flow at a certain frequency, and a movable iron core is slid finely by changing a ratio between ON time and OFF time of the current, or a dither control wherein the movable iron core is finely slid, in which a certain DC voltage is provided with a variation (AC component), is generally utilized.
For instance, when a rated voltage which is applied to the linear solenoid is determined to be 12V as a reference, in case of the duty control, the applied voltage to the electromagnetic coil is 12V in case of the duty ratio of 100%. In case of the duty ratio of 50%, a mean applied voltage is 6V. The target flow quantity is determined corresponding to the duty ratio, since the position of the movable iron core is determined corresponding with these values. When the voltage applied to the linear solenoid is changed by a variation of the battery voltage, the target flow quantity can be obtained by changing the duty ratio so that the variation is corrected. To obtain the same flow quantity the correction is performed by the following equation. EQU D.sub.EV =D.sub.12V .times.12/E
where E is the terminal voltage of the linear solenoid, D.sub.EV, the duty for the terminal voltage E(V), and D.sub.12V, the duty for the terminal voltage 12V. The correction with respect to the coil temperature of the solenoid can be considered in the same way. For instance, when a coil resistance R.sub.20 at 20.degree. C. is determined as a reference, a coil resistance R.sub.t at t.degree. C. is shown by the following equation.
R.sub.t =R.sub.20 .times.{1+.alpha.(t-20)},
where .alpha. is temperature coefficient of resistance (1/deg) of copper wire. Accordingly, when the coil temperature is at t.degree. C., to obtain the flow quantity which is the same as that at 20.degree. C., the correction is to be performed by the following equation. EQU D.sub.t =D.sub.20 .times.R.sub.t /R.sub.20,
where D.sub.t is the duty at t.degree. C., and D.sub.20, the duty at 20.degree. C.
When the linear solenoid is utilized in the idling revolution number control, normally, the above correction with respect to the voltage or the temperature is performed. Especially in case of the correction with respect to the coil temperature, a method wherein the duty ratio is corrected by detecting the value of the electric current which flows in the coil, causes an increase of size of the ECU or an increase of production cost. Therefore a method is much used wherein a portion in which cooling water of the engine is flown adjacent to outside of the coil of the linear solenoid is provided, the coil temperature is equalized with a cooling water temperature of the engine, and the coil temperature is represented by the cooling water temperature which is detected by a cooling water temperature sensor (normally always equipped). In this case, to improve thermal transfer between the coil and the cooling water, an outer structure of the linear solenoid is made by aluminum die casting or the like, wherein a cooling water passage is constructed, and a space among the aluminum die casting structure, the coil and magnetic circuit parts, is filled with resin or the like.
FIG. 5 shows a construction diagram of the conventional idling revolution number control valve, wherein a reference numeral 1 designates a solenoid case made by the aluminum die casting, 2, a fixed iron core, 3, a case which accommodates a magnetic circuit, 4, a movable iron core, 5, a return spring which pushes the movable iron core to the reverse side of the fixed iron core 2, 6, an electromagnetic coil which is provided outside of the fixed iron core 2 and the movable iron core 4, and 6b, a takeout wire provided from the electromagnetic coil 6 to the outside thereof. A reference numeral 4b designates a through hole which penetrates the central portion of the movable iron core 4, 14, a valve which is provided at the back end of the movable iron core 4, and 13, a valve seat which is utilized in opening and closing operations of the valve 14.
A reference numeral 11 designates a main body of the idling revolution number control valve, 9, a spring holder which is fixed to the main body 11 through an attaching screw 10, 8, a spring which is provided to the movable iron core 4 through a holder 16 or the like, 15, a spring provided between the movable iron core 4 and the valve 14, 11a, a fluid flow-in passage, 11c, a fluid flow-out passage, and 11b, a fluid chamber which communicates both passages 11a and 11c. A reference numeral 20 designates a cooling water passage which is formed in the solenoid case 1. FIG. 6 shows a construction diagram of another example of the conventional idling revolution number control valve, wherein since the construction is the same as in FIG. 5, the same notations are given thereto and further explanation is omitted.
Since in the conventional idling revolution number control valve, the solenoid case 1 is made of the aluminum die casting structure and the cooling water passage 20 is provided therein, it is difficult to downsize it and the production cost thereof is high. The shape of the cooling water passage which is composed by the aluminum die casting structure depends on the kinds of vehicles due to piping layouts thereof. Therefore a matching of the coil temperature and the cooling water temperature causes an increase in production steps in development of a new model car.
Furthermore, it is normally important to correct the change of the coil temperature in driving the valve in an idling state of the engine after the warming-up of the engine. Moreover, the cooling water passage of the aluminum die casting structure is not utilized under a condition wherein flow quantity accuracy is not required.